So, if you think about sugar, this is a more complicated structure. It's made up of 3 different types of atoms - we've got carbon, hydrogen, and oxygen. And if we warm this up, well, we can get liquid sugar. We can melt the sugar, and this is where the molecules are all moving around, still in their sugar forms. But if you heat that up too much, again, different parts of the sugar molecules might start to interact with each other and form new chemical compounds. You can boil off water and you're going to be left with this sort of black gooey mess which is carbon.

Now, when you come to a potato, it's even more complicated. You haven't got one type of substance there. You've got complete array of all sorts of complexity. You've got water there, you've got starches in there, you've sugars in there, you've got fats in there. All of these can start interacting with each other. You're not just going to end up with one liquid mess of all of those. They're going to start interacting at much lower temperatures, recombining, giving out water vapour and decomposing what's left into this sort of black gooey carbon mess, and that's what you end up with - your burned potato.

Hannah - So, due to the chemical complexity of the simple starchy spud, it can't melt. Instead, cooking causes the constituent molecules in the potato to interact, evaporate and decompose, and at really high temperatures, form just black carbon. Marshmallow on the other hand is made of simpler smaller molecules including refined sugars which, if we carefully heat at low temperatures, we can melt the inside and keep the sugars intact.